1. Field of the Invention
The invention relates to a process for unloading transport vehicles containing a liquified gas. More particularly, the invention relates to a process that uses the latent heat conversion energy characteristics of certain gases such as carbon dioxide or nitrous oxide in their solid state to unload and store vapor remaining in rail cars or trucks after a liquified gas has been unloaded.
2. Brief Description of the Related Art
Liquified gases such as liquid carbon dioxide and liquid nitrous oxide are shipped to customers or depots as refrigerated liquids in insulated railroad tank cars. The shipping temperatures for liquid carbon dioxide range, for example, from 150 psig, -34.degree. F. (10.34 bars, -36.7.degree. C.) to 350 psig, +11.degree. F. (24.13 bars, -11.7.degree. C.). The railroad cars used for shipping liquified gases typically do not have refrigeration, thus, the liquified carbon dioxide or other liquid increases in pressure during transit due to normal warming of the liquid via heat transfer through the insulation of the rail car. A typical shipment by rail takes 5-20 days depending on both the distance traveled and the number of rail transfers required. Ambient heat entering the insulated rail car during transit gradually warms the liquified carbon dioxide increasing the pressure inside the rail car. A relief valve is provided on the rail car and set to operate at about 350 psig (24.13 bars) to vent a small amount of vapor carbon dioxide to the atmosphere to self refrigerate and maintain the pressure within the car at 350 psig (24.13 bars).
Although all attempts are made to reduce or eliminate venting losses during transit due to warming of the liquid, the internal pressure on a rail car arriving at an unloading location is often as high as 350 psig (24.13 bars). At the unloading location, the liquid carbon dioxide is removed from the rail car and transferred to a delivery tanker, storage tank, or depot tank. Most depot tanks maintain storage pressures of between 200 psig, -20.degree. F. (13.79 bars, -28.9.degree. C.) and 300 psig, 2.degree. F. (20.68 bars, -16.7.degree. C.). The depot tank pressure is controlled by a mechanical refrigeration system that cools and condenses carbon dioxide vapor to achieve the desired depot tank pressure. Rail cars may also be unloaded directly into delivery tankers for delivery to a final destination. Most carbon dioxide delivery tankers have design pressures of between 250 psig (17.24 bars) and 300 psig (20.68 bars). Thus, it is not possible to pump "warm" high pressure carbon dioxide directly from the rail car at 350 psig (24.13 bars) into the delivery tankers, storage tanks, or depot tanks without first decreasing the rail car pressure.
The rail car pressure can be decreased either 1) by venting vapor to the atmosphere; 2) by using mechanical refrigeration to cool liquid and condense vapor removed from the rail car; or 3) by mixing cool carbon dioxide liquid in a depot tank with the warm liquid and/or vapor carbon dioxide from the rail car to equalize the liquid carbon dioxide at an acceptable pressure. Generally, venting of the carbon dioxide vapor to the atmosphere to reduce the rail car pressure is undesirable since venting losses decrease efficiency. Therefore, refrigeration or a combination of refrigeration and mixing with cold liquid are generally used to decrease the rail car pressure to an acceptable level.
A typical rail car contains approximately 80-90 tons (72,570-81,645 kg) of liquid carbon dioxide. Once the liquid carbon dioxide is unloaded from the rail car, there is approximately three to four tons (2720-3630 kg) of carbon dioxide vapor left in the car at about 300 psig (20.68 bars) to 350 psig (24.13 bars). Typically, a compressor is used to remove some of this high pressure carbon dioxide vapor from the rail car and increase the pressure of the vapor sufficiently to force it into the depot tank. A refrigeration system associated with the depot tank, then condenses the vapor to a liquid to maintain the normal tank pressure of 200 psig (13.79 bars) to 300 psig (20.68 bars). However, this process requires that the refrigeration unit of the depot tank have sufficient capacity to condense the vapor at the same rate as it is extracted from the rail car. The refrigeration unit must be large enough to handle ordinary heat leak through the depot tank insulation, the entire heat load of the warm liquid carbon dioxide from the rail car, and the heat of condensation for the vapor which has been extracted from the rail car.
The process of unloading an approximately 80 ton (72,570 kg) rail car typically takes between 4 and 8 hours, and the amount of heat that must be removed from the storage tank to maintain the required storage tank pressure and prevent vapor from being vented is approximately 2.times.10.sup.6 Btu/rail car. This is equal to approximately 21 tons (15.2.times.10.sup.5 Cal) of refrigeration spread over 8 hours. In contrast, the refrigeration which is required to maintain the depot tank pressure and compensate for normal heat leak through the depot tank insulation is typically less than 5 tons (3.6.times.10.sup.5 Cal) for the same 8 hour period.
Another method for reducing the temperature and thus, the pressure of the liquid carbon dioxide in the depot tank is to maintain a cool supply of liquid carbon dioxide within the depot tank and deliver the warm carbon dioxide liquid from the rail car to the depot tank mixing the hot 350 psig, 11.degree. F. (24.13 bars, -11.7.degree. C.) rail car liquid with cool 200 psig, -20.degree. F. (13.79 bars, -28.9.degree. C.) stored liquid to chill the hot rail car liquid. Typically, depot storage tanks have a minimum design metal temperatures (MDMT) of -20.degree. F. (-28.9.degree. C.) which is the lowest liquid temperature which can be safely used with the depot tank without the metal becoming brittle. This means that the lowest temperature allowed for the cool carbon dioxide liquid maintained in the depot tank to be mixed with the hot rail car liquid would be 200 psig, -20.degree. F. (13.79 bars, 28.9.degree. C.). Therefore, if cold depot liquid is going to be mixed with a warm rail car liquid to reduce the required refrigeration load at the time of unloading the rail car, then 200 psig, -20.degree. F. (13.79 bars, -28.9.degree. C.) is effectively the practical and economic low temperature limit for the cold depot liquid. Accordingly, the process of cooling hot rail car liquid with a supply of cold liquid in the depot tank works only when there is an adequate volume of cold liquid to equilibrate at an acceptable temperature level. If the mass of cold liquid in the depot tank is low, then there is little energy that can be "borrowed" from the cold liquid to chill and equilibrate with the hot rail car liquid unloaded from the rail car.
A problem that users and manufacturers of carbon dioxide and other related liquified gases face is to be able to install refrigeration units on the depot tanks which are large enough to recover all of the liquid carbon dioxide and most of the vapor carbon dioxide without requiring venting to the atmosphere or returning the car partially filled with carbon dioxide vapor. The refrigeration unit which is required to handle the entire heat load of an approximately 80 ton (72,570 kg) rail car must be able to cool 2.times.10.sup.6 Btu/rail car during the 4 to 8 hour unloading time period. In addition, United States Department of Transportation regulations require that rail cars be attended at all times during unloading. Therefore, in order to reduce the cost of labor, it is economically desirable to unload rail cars as rapidly as possible. This means that the refrigeration unit needs to be of a sufficient size to handle the large instantaneous cooling load. Otherwise, not all of the available vapor can be recovered before the rail car is returned to be refilled. The large and expensive refrigeration unit required to achieve the desired unloading time of between 4 and 8 hours is generally underutilized during a substantial portion of time when rail cars are not being unloaded. Further, most rail car unloading is performed during daylight hours which correspond with on-peak electric power rates.
Accordingly, it would be desirable to provide a system for unloading rail cars at the same or a faster rate than is currently possible, while using a smaller refrigeration unit. It would also be desirable to be able to operate the refrigeration unit during off-peak hours when electric power rates are lower and to still be able to unload the rail car during daylight hours.